This invention relates to the art of vapor deposition processes, and more particularly to an improved apparatus for sensing and monitoring the deposition rate and composition of the vaporized particles in the vacuum chamber.
A variety of deposition rate monitors have previously been evolved. However, these have proven generally inadequate, particularly when applied in heavy duty production systems.
Thus, quartz crystal detectors have been employed with a quartz crystal arranged in the vacuum deposition chamber, with a sensing face of the crystal on which the particles in the vapor in the system condense. The natural frequency of oscillation of the quartz crystal is effected by the condensation of these particles on the crystal, and the change in oscillation frequency is indicative of the mass of the condensed material. Such quartz detectors are, however, subject to a limited useful life as a result of the accumulated coating of the crystal by the evaporant in the monitored deposition chamber.
Other monitoring techniques have been developed based on ionization of the evaporant particles by electron bombardment in the deposition chamber. The magnitude of the resulting ion current is a measure of the deposition rate. This method requires very sensitive measurement of weak ion current in the deposition chambers and thus is highly susceptible to electrical noises. It is also incapable of distinguishing different materials.
Additionally, in vacuum deposition systems employing electron beam gun heated evaporation sources, monitoring techniques have been evolved relying on observing the intensity of emitted light in the ionized vapor cloud directly above the evaporation source by the same electron beam gun which produces the evaporant vapor. Due to the rapidly fluctuating vapor density near the evaporation sources, the uncontrollable state of ionization, and scattered light from many possible sources, variations in light intensity are found to produce inaccurate readings.
Others have attempted to bombard particles in the deposition chamber to cause emission of X-rays, which are characteristic of the particular particles. The voltage required to produce emission of these X-rays, and the shielding necessary to protect the users of the equipment are such as to make such techniques commercially unfeasible.